In recent years, there has been interest in the polar modulation method as a technology for achieving both high linearity and high efficiency in a transmit modulator (e.g., see Patent Literature 1). FIG. 15 is a diagram showing an exemplary configuration of a transmit modulator using a conventional polar modulation method (hereinafter, referred to as a polar modulation apparatus). In FIG. 15, the conventional polar modulation apparatus includes a power amplifier (PA) 51, which amplifies a phase modulated signal, and a power supply control unit 52, which controls a power supply voltage of the power amplifier 51, based on an amplitude component signal. The phase modulated signal is input to the power amplifier 51. The amplitude component signal is input to the power supply control unit 52. As described above, the employment of the polar modulation method allows the phase modulated signal, which is input to the power amplifier 51, to be a constant envelope signal which has no fluctuation component in the amplitude direction. This allows the usage of a non-linearity amplifier operable at high efficiency as the power amplifier 51. On the other hand, it is required that a proportional relationship is established between a voltage value of the amplitude component signal and an output voltage of the power amplifier 51.
Also, an HBT (Hetero-junction Bipolar Transistor) device, which allows higher gain than an FET device and which can be readily miniaturized, is used as an element for use in the power amplifier 51. In the HBT device, however, a band offset occurs at an interface between dissimilar semiconductor layers being overlapped one on the other, and therefore there is a specific parameter called an amplitude offset voltage between the power supply voltage and the output voltage.
FIG. 16 is a diagram illustrating the amplitude offset voltage of the power amplifier 51. The horizontal axis in FIG. 16 indicates a power supply voltage Vcc of the power amplifier 51, and the vertical axis indicates an output voltage Vout of the power amplifier 51. As shown in FIG. 16, if the HBT device is used as the power amplifier 51, although the power supply voltage Vcc and the output voltage Vout change linearly, the line does not pass the origin, and therefore does not represent the proportional relationship. That is, the relationship between the power supply voltage Vcc and the output voltage Vout is non-linear near the origin. Unless an appropriate correction is made in the non-linear region, the relationship between the power supply voltage Vcc and the output voltage Vout deviates from its ideal characteristic, and the amount of the deviation appears as a distortion component. In this case, the problem is effects on an adjacent channel and a receive band, and these are strictly regulated by standard.
Due to the above-mentioned reason, the amplitude offset voltage needs to be considered in the case of using the HBT device as the power amplifier 51. The amplitude offset voltage is a power supply voltage Vcc at the time when the power amplifier 51 output rises. For example, the relationship between the power supply voltage Vcc and the output voltage Vout can be linear fit as shown in FIG. 16 to regard a voltage Vcos at a point of intersection between the linear line and the Vcc axis as the amplitude offset voltage. An adder 53 adds the amplitude offset voltage to the amplitude component signal, thereby avoiding the occurrence of distortion at the power amplifier 51.
In addition, it is assumed that an amplitude offset voltage, which is actually added to the amplitude component signal, is Vos, and an amplitude offset voltage that is optimum for the power amplifier 51 is Vamo. FIG. 17 is a diagram illustrating the optimal amplitude offset voltage Vamo. The definition of the optimal amplitude offset voltage Vamo can be determined arbitrary depending on the specification required for a system. Here, an amplitude offset voltage obtained when an Adjacent Channel Leakage Power Ratio (ACLR) is a minimum is defined as Vamo, as shown in FIG. 17. Alternatively, an amplitude offset voltage obtained when receive band noise or EVM (Error Vector Magnitude) is a minimum may be defined as Vamo.
It is known that there is strong correlation between the Vamo and the Vcos which are thus defined. Therefore, calculating and compensating for the Vcos of the power amplifier 51 allows suppression of the output signal distortion, and suppression of an adjacent channel leakage power ratio (ACLR) and the receive band noise within desired ranges, respectively.